The objective of this proposal is to continue our studies on understanding the mechanisms of tumor necrosis factor-alpha (TNF??) signaling in cystogenesis. TNF?? through its receptor and through the ability of the receptor associated proteins to form the membrane bound pro-survival complex I as well as the cytoplasmic pro-death complex II help cancer cells to escape apoptosis. The inhibitor of apoptosis (IAP) antagonist, Smac-mimetic, is able to target the above TNF??-mediated complexes to induce cancer cell death. Our studies have suggested that TNF?? can promote ADPKD progression and our preliminary studies have further found: 1) TNF?? is always present in cyst fluid from ADPKD kidneys; 2) Loss of polycystin function leads to increased expression of TNF?? receptor I (TNFR-I); 3) The components of proteins in complex I and complex II are upregulated in Pkd1 mutant MEK cells; 4) Most importantly, TNF?? alone could not induce normal or Pkd1 mutant epithelial cell death even at high concentrations, while, TNF?? together with transiently transfected Smac could induce only Pkd1 mutant cystic epithelial cell death but had no effect on Pkd1 wild type kidney epithelial cells. Furthermore, the following evidence supports the connection between TNF?? and Id2 signaling pathways: 1) TNF?? activates mTOR through IKK? and mTOR regulates the functional differentiation of mammary epithelial cells through regulating the expression of Id2; 2) Receptor activator of NF-?B ligand (RANKL), a TNF family molecule, regulates mammary epithelial cell proliferation via Id2 by triggering marked nuclear translocation of Id2. Based on these findings, we hypothesize that: TNF?? and its receptor are able to regulate cystic epithelial cell apoptosis through the membrane bound pro-survival complex I and cytoplasmic pro-death complex II and further regulate cystic epithelial cell proliferation and differentiation through mTOR or RANKL-mediated Id2 signaling. To test our hypothesis, we propose three aims: 1) To Investigate the mechanism(s) of TNF?? signaling in regulation of kidney epithelial cell survival; 2) To determine the potential role and mechanism of the IAP antagonist, Smac-mimetic, in regulating TNF??-dependent cystic kidney epithelial cell death; 3) To investigate whether TNF?? signaling regulates cystic epithelial cell proliferation and differentiation through mTOR and RANKL mediated Id2 signaling. Accomplishing this project will further elucidate the role of the pathways downstream of the polycystins in cystogenesis and will identify key regulatory components that may serve as effective targets to slow disease progression.